A student wants to demagnetise a permanent steel bar magnet. They design four different experimental procedures involving a solenoid:
- Procedure 1: The magnet is placed inside a solenoid connected to an alternating current (a.c.) supply, and then the magnet is slowly withdrawn until it is far away from the solenoid.
- Procedure 2: The magnet is placed inside a solenoid connected to a direct current (d.c.) supply, and then the magnet is slowly withdrawn until it is far away from the solenoid.
- Procedure 3: The magnet is placed inside a solenoid connected to an alternating current (a.c.) supply, and the current in the solenoid is slowly reduced to zero using a variable resistor.
- Procedure 4: The magnet is placed inside a solenoid connected to a direct current (d.c.) supply, and the current in the solenoid is slowly reduced to zero using a variable resistor.
In which of these procedures is the magnet successfully demagnetised?